Blood moves most slowly in which type of blood vessel?

In your capillary bed -- this is where the blood flows from a tube (your vascular system) into your tissue. Because blood is being essentially dumped out, it's important to regulate the amount flowing out (rings of smooth muscle around adjacent arterioles prevent too much blood flow).

This is important -- that blood FREELY DIFFUSES across a 1-cell-thick capillary. What this means, is that blood will flow without an outside force towards the tissue (slowly) to allow enough time for gas and nutrient exchange.

My answer is extremely un detailed and I've left many points out. You can see the website yourself.

Arteries carry oxygenated blood under high pressures - so they have thickened muscular walls and small centres (lumens) and no valves. Veins carry slower moving deoxygenated blood so they have thinner walls and contain valves to prevent back flow of blood. Capillaries are designed to deposit the oxygen in the remotest parts of the body. They have thin walls - up to only 1 cell thick to allow diffusion of oxygen into tissues and waste carbon dioxide back into the blood.